Thanks all for your help.
It did turn out that removing BT McAfee and installing M/S Essentials solved the problem.
I have no idea why. I could find no reference to that for this specific problem on other websites, but as ppruners know, some folk are very anti-McAfee. I just use it cos BT gives it free, and cos it has (so far) been OK for my pc.s. - Although my new one only has pprune recommended M/S Essentials and firewall. I also seldom use an admin i.d.
Once I've "saved" daughter's stuff, I will do as Milo suggests and have a factory reset......but as I've never done that before I'll worry about that next week.